On March 10, a choir in Washington State decided to go ahead with practice amid increasing reports of the coronavirus spreading within the U.S. This rehearsal, attended by 61 of the choir’s usual 122 members, lasted for two and a half hours, with attendees taking precautions not to mingle too much. Ultimately, an estimated 87% of attendees later got sick, according to a recent CDC report on the event.
“The choir practice is one of our best examples of a super-spreading type of event,” says Prathit Kulkarni, M.D., an infectious disease specialist at Baylor College of Medicine. A super-spreading situation is defined as when a single person infects a large number of other people in a single event.
The choir practice is a primary example of super-spreading during the current pandemic, but there have been others—notably gatherings on a cruise ship and at a funeral, an international conference and in a women’s prison. While in these cases it’s harder to pinpoint as clearly one ill person who triggered the outbreak, they still qualify, as we know that all of them led to a large number of infections.
As scary as a super-spreading event sounds, they are already rolled into the average rate of transmission. A few infected people may get get a lot of people sick—such as during the choir practice—while most people will only get one or two sick. The average estimated rate of transmission of a virus is known as its R0. The coronavirus’s R0 is estimated to be between 2.0-2.5, which means, on average, one person will infect between 2.0 to 2.5 other people, with super-spreading events factored into this number.
A high density of people in an enclosed area increases risk of transmission
As Kulkarni notes, it’s the density of people in an enclosed area and/or the length of time they are in that enclosed area that leads to an increased risk of infection. During the choir practice, a large number of people were in a confined room together for a period of several hours.
On the Diamond Princess cruise, 700 out of a total of 3,711 passengers and crew members got sick—another environment in which everyone was confined to one space (albeit a much larger, more compartmentalised one). The same is true for a women’s prison in Louisiana, where almost all the inhabitants of one particular dormitory tested positive for COVID-19.
“In these closed settings, when people are together for long periods of time, there is definitely a chance of a large number of people potentially getting infected,” Kulkarni says. Generally speaking, the longer people are together, the more chance they have of being infected, whether it is because of exposure to respiratory droplets during a face-to-face conversation, shaking hands with an infected person or touching a contaminated surface.
Biogen conference: Two days of mingling, with attendees returning home
Another notable example, which occurred in late February, when there were still relatively few cases in the U.S., was the Biogen conference in Boston, which ultimately led to an estimated 99 cases among both attendees and people they later came into contact with.
The infections stemming from the conference, during which a large number of attendees met repeatedly over two days, were made more complicated by the fact that attendees went back to their own communities, both within the U.S. and around the world, where they got others sick.
As Kulkarni notes, the Diamond Princess cruise, the Louisiana prison and the Biogen conference are variations of a super-spreading event, even though all of the cases can’t be traced back to one single infected individual.
Isolation and quarantine are effective measures for limiting transmission
The good news? Although a large percentage of the attendees of the choir practice got sick, it did not make a significant impact on transmission within the larger community, in large part because all of the attendees acted quickly, initiating their own isolation and quarantine even before health officials got involved. Measures such as hand-washing, wearing masks, practicing physical distancing, quarantining in the event of exposure and isolating in the event of developing symptoms are effective at limiting the overall infection rate—even after a super-spreading event.
“These are still the best practices and best mechanisms we have to try and prevent spread,” Kulkarni says.